1. Field of the Invention
The present invention relates to a method for fabricating a glass preform for optical fiber having a silica glass portion doped with fluorine, which has a step of performing deposition of fine glass particles by the vapor phase axial deposition (VAD) process.
2. Related Background Art
Optical fibers have the structure of the core and the cladding having a lower refractive index than the core and surrounding the core. Utilizing the refractive index difference between the core and the cladding, light is transmitted as confined in and near the core.
Thus, in order to achieve the function as an optical fiber, a dopant for raising or lowering the refractive index is added to a base material, thereby realizing a desired refractive index profile. If the base material is silica glass, fluorine is suitably used as a dopant for lowering the refractive index.
The optical fiber having the silica glass portion doped with fluorine is fabricated by heating and drawing a glass preform for optical fiber having the silica glass portion doped with fluorine. Such glass preforms for optical fiber have been fabricated heretofore as follows.
The first conventional method for forming the silica glass portion doped with fluorine (hereinafter referred to as conventional example 1) is arranged, for example as disclosed in Japanese Patent Publication No. 62-38292, in such a manner that, for growing a body of fine glass particles by the VAD process, glass raw material gas and combustion gas are supplied and glass particles are deposited in the direction of target axis by flame hydrolysis and thereafter a consolidating step of glass particles is carried out under an atmosphere of fluorine compound gas, thereby obtaining a glass preform for optical fiber having the glass portion doped with fluorine.
The second conventional method for forming the silica glass portion doped with fluorine (hereinafter referred to as conventional example 2) is arranged, for example as disclosed in Japanese Laid-open Patent Application No. 59-232934, in such a manner that, for growing a body of fine glass particles by the VAD process, the both glass raw material gas and combustion gas, or only the combustion gas containing the fluorine compound gas in a concentration kept constant in terms of the time is supplied and fine glass particles are deposited in the direction of target axis by flame hydrolysis and thereafter the fine glass particles are consolidated, thereby obtaining a glass preform for optical fiber.
The following methods are proposed as methods for making fluorine doping concentrations uniform mainly in the radial direction in the glass preform for optical fiber the base material of which is silica glass.
In the method disclosed in U.S. Pat. No. 5,039,325, first, the fluorine compound gas is mixed in the glass raw material gas and fine glass particles are deposited by the outside vapor deposition process. Next, the volume of a heating furnace is set slightly larger than that of a deposit body of glass particles and heating is effected on the deposit body as decreasing supply amounts of the atmospheric gas upon heating so as to transparentize the deposit body. As a result, a vaporization amount of fluorine is decreased from the peripheral portion upon the heating transparentization, thereby obtaining the glass preform for optical fiber with the fluorine doping amounts uniform in the radial direction.
In the method as disclosed in Japanese Laid-open Patent Application No. 61-236626, first, fine glass particles are deposited using a mixture of the fluorine compound gas mixed in the glass raw material gas in lower concentration as approaching the peripheral portion. Next, the deposit body is heated to be transparentized under an atmosphere containing fluorine or a fluorine compound. As a result, differences of fluorine doping amounts in the radial direction upon deposition of glass particles are canceled by differences of fluorine doping amounts in the radial direction upon heating transparentization, thereby obtaining a glass preform for optical fiber with fluorine doping amounts uniform in the radial direction.